This invention relates to an improved process for forming optical preforms for the production of optical fibers. The process is especially suitable for making optical preforms which possess step or graded refractive index profiles. Such characteristics enable the production of optical fibers cables exhibiting reliable operating characteristics.
There has been a continuous search in the prior art for the economical and mass production of fiber optic cables for use in optical communications systems.
Thus, the prior art considered and describes techniques such as "soot" deposition or hydrolysis wherein a gas vapor mixture is hydrolyzed by a flame to form a glass precursor particulate. The particulate is then deposited on a rotating glass rod serving as a mandrel. The soot is deposited upon the mandrel in a perpendicular direction to provide successive layers of constant radius or to provide a composite article with radial gradations in the index of refraction by varying the dopant concentration during successive passes of the burner flame. The mandrel is then removed and the thus released cylindrical article is collapsed to a solid rod-shaped preform and then a fiber is drawn from this preform. This process is shown and discussed in U.S. Pat. No. 3,826,560 and U.S. Pat. No. 3,823,995. This process, however, is very laborious and time-consuming. Consequently, the preform and the fiber drawn therefrom are relatively expensive. Moreover, it is very difficult if not impossible to achieve a complete utilization of the soot or of the material from which it is obtained because of difficulties encountered in capturing and/or recycling such materials.
Other techniques, such as that described in U.S. Pat. No. 3,614,197 involve processes for continuously forming an optical fiber by using a multi-stepped funnel-shaped vessel to form a solid glass rod-shaped preform which is then heated and drawn into a fiber. Even this procedure is rather expensive and prone to result in contamination of the preform and the fiber by undesirable inclusions.
In any event, there is a desire to provide a solid optical preform and then draw or process the same into an optical fiber. Both the continuous forming process and the tubular preform forming and collapsing approach have inherent benefits in the mass production of such cables but also certain disadvantages.
Furthermore, U.S. Pat. No. 3,966,446 discusses another technique for providing an optical preform. The optical preform is here fabricated by the axial deposition from a direction along the preform axis as opposed to radial deposition from a direction perpendicular to the preform axis as used in the above-mentioned approaches. This technique does not require a mandrel and thus avoids the need for collapsing a cylindrical preform prior to drawing. Yet, even this technique is rather cumbersome and time-consuming and, consequently, expensive. In most instances, the avoidance of the need for collapsing the preform is more than outweighed by the inconvenience of using such a complicated process and the expense associated therewith.
The preforms thus provided in the just mentioned patent may be provided with longitudinal gradations in the index of refraction and thus serve to enhance certain types of mode conversions. However, this technique is not readily suited for providing radial gradations in the index of refraction. This is an additional reason for not using this approach in the fabrication of fiber with radially graded index of refraction.
In any event, there is a need to provide large optical preforms which then can be drawn into elongated optical fibers. There is a further need to provide an optical preform which can exhibit step, single mode or graded index profiles in the radial direction to enable the resultant cable to be used to more efficiently transmit optical information in the form of digital or other signals.
It is known that optical fiber cables which possess a single mode of operation alleviate mode dispersion problems. It has been a problem to produce reliable cables employing single mode operation in that the prior art techniques could not adequately control the composition of the cable. Moreover, it is difficult to assure that the operation will be conducted in the single mode under all operating conditions. Thus, many cables employ a multi-mode operation in using radial gradations in the index of refraction. In these cables the difference in velocity from mode to mode compensates for the different path lengths and results in a relatively equal traversal time for all modes.
It is clear that, in order to efficiently employ a single mode or a multi-mode operation, one must carefully and accurately control the fabrication of the fiber to assure that the same is consistent in formulation and hence, possesses repeatable and reliable operating characteristics.
The current fabrication techniques of all optical fiber preforms are broadly based on one fundamental principle, i.e. vapor phase deposition. For example, the reported processes are chemical vapor deposition (CVD), modified chemical vapor deposition (MCVD), outside vapor phase oxidation (OVPO), inside vapor phase oxidation (IVPO), and plasma-activated chemical vapor deposition (PCVD). In all these processes, metal halides, such as pure or doped silicon halides, are converted at high temperatures to the respective oxide particles and the chemical conversion and deposition processes occur simultaneously. As mentioned before, such conventional processes are rather time-consuming and expensive, especially because of the slow rate of growth of the deposited layer and the need for performing such processes in carefully controlled atmospheres and at relatively high temperatures.